Powered shield supports have been used successfully for some time in underground extraction operations of bituminous coal. These are designed as so called lemniscate shields and are generally fitted with two or four hydraulic rams engaging under the canopy. However, these powered shield supports have to be of extraordinary stable design in terms of their components, in particular, their canopy, floor skid, guide bars and various hinges, so that they are able to cope with difficult conditions of use and unfavourable loading situations. This leads to a heavy and correspondingly expensive construction of the powered shield supports.
From the point of view of the mine operator, for reasons of application and economics, there is considerable interest in restricting the weight and hence also the costs of the shield construction. Depending on the existing infrastructure of the mines, and also on the seam strengths which are found, it is often possible only to use powered shield supports whose weight does not exceed about 15 t to 30 t. This limitation on the weight leads to high-strength and correspondingly expensive steel plates and steel cast parts having to be used for the highly-loaded components of the powered shield supports, which leads to considerable increases in costs in the production of the powered shield supports. In spite of the use of high-strength materials to restrict the weight, overloading of individual components occurs frequently during the underground use of the powered shield supports. Hence, high repair costs and a reduction in the service life of the powered shield supports.
In recent times, in order to reduce the investment and operating costs, powered shield supports have been used whose centre-to-centre spacing or overall width is 1.75 m instead of the previously usual dimension of 1.5 m. Further optimisation could be achieved using powered shield supports with even greater overall widths, but these would result in the abovementioned weight limitations being exceeded.
In modern support technology, the shield support, as is known, is equipped with electrohydraulic control systems, namely an electronic controller equipped with a microprocessor in each powered shield support. In this situation, the sensors are also used for detecting the respective ram pressures and the advancing cylinder strokes. These sensors being connected to the controller by their electric signal lines. Sensor technology is primarily used here for the automatic control of the movement sequences and tracking the powered shield support and the face conveyor and, if appropriate, also for monitoring the ram pressures.
Many attempts have been made in the past to construct the shield support more lightly and to design it such that its highly-loaded components are protected against overload and damage. DE 31 41 040 C1 proposed constructing the front guide bar or guide bars of the lemniscate mechanism as a hydraulic guide bar in the shape of a hydraulic cylinder. The intention being to keep the guide-bar forces constant or protected against overload during the use of the powered shield support. However, this solution path has not become widespread in practice, particularly because of the associated higher costs and the limitation in the force of the hydraulic guide bar due to the limitation in its cylinder diameter.
Further solution proposals for reducing the weight of the powered shield supports are indicated in the magazine "Gluckauf" 1982, pp. 927 to 933. Here, too, the use of hydraulic guide bars for limiting the external forces parallel to the stratum is proposed. In addition, a reduction in weight is intended to be achieved in that, in the case of inclined upright rams, the ram pressures are controlled as a function of the ram angle, in order to keep the shield support force constant over the height adjustment range, or to cut off or to suppress peak values in terms of loading which result in the case of upright rams.